There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
nig-
verb. to steal, creep, do or go by stealth
nig(la)
noun. mouse
The words G. nig and nigla “a mouse” appeared in the Gnomish Lexicon of the 1910s above the verbs G. nigla- “thieve” and G. nig- “steal” (GL/60), probably derived from a variant of the early root ᴱ√NAQA “steal” (QL/64).
Neo-Sindarin: I would adapt this word into Neo-Sindarin as ᴺS. nigol “mouse”, reconceived of as elaboration of √NIK “small”.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A verb for “steal, creep, do or go by stealth” in the Gnomish Lexicon of the 1910s with past form naigi (GL/60), probably based on the early root ᴱ√NAQA “steal, take; get by stealth, unlawfully” or a variant thereof (QL/64).